1. Field of the Invention
The present invention relates to an improved method of fabricating an electric lamp to substantially eliminate the presence of trace impurities within the lamp envelope thereby improving lamp life.
2. Description of the Prior Art
Most lamps in use today are adversely affected by the presence of oxygen or water vapor in the internal lamp atmosphere. This is a particular problem regarding high voltage lamps but can also be a concern with respect to low voltage lamps. Incandescent lamps suffer shortened lamp life and blackening of the inner surface of the lamp bulb as a result of the presence of oxygen and especially water vapor by the well known tungsten-transporting water cycle. This problem is of concern, for example, in a conventional tungsten halogen incandescent lamp. Similarly, fluorescent lamps are subject to erosion of any uncoated cathode wire, and cathode coating emissivity and life are adversely affected by traces of oxygen or water vapor within the lamp. This is well documented in the publication "Poisonous Gas Effects On The Emission Of Oxide-Coated Cathodes", S. Itoh, M. Yokoyama, and K. Morimoto, J. Vac. Sci. Technol., A5(6), pp. 3430-3435, Nov./Dec. 1987. In addition, the presence of hydrogen in a fluorescent lamp can also damage the phosphor and increase lamp starting voltage. Such hydrogen typically is formed in a fluorescent lamp as a result of the dissociation of water vapor.
Incandescent lamps typically contain chemical getters which combine with oxygen or water vapor to remove such impurities from the lamp atmposphere. In addition, the tungsten filament of an incandescent lamp can be freed of any oxides formed, for example, during the sealing of the lamp envelope, by a brief light-up in the presence of a forming gas. This process is effected prior to the final exhausting of the lamp vessel and introduction of the desired fill gas. Typically the forming gas is a mixture of hydrogen in nitrogen. During such light-up procedure, the capsule is given one or more light-ups at the desired voltage while filled with the mixture of hydrogen and nitrogen. The hydrogen chemically reduces any tungsten oxide present to form tungsten metal and by-product water vapor which is then pumped out of the lamp.
It is desired to improve lamp component deoxidation and thereby improve lamp life. It has been observed that such improvement can be accomplished using a mixture of carbon monoxide in nitrogen in place of the hydrogen-containing forming gas.
The use of carbon monoxide in the processing of incandescent lamps is known in the art. However, to date such use has been directed to providing carbon monoxide as a fill gas to be used as an oxygen source. For example, in U.S. Pat. No. 3,364,376 to Collins et al., which issued on Jan. 16, 1968, an iodide cycle incandescent lamp is provided wherein carbon monoxide is a component of the fill gas to enhance the halogen cycle to prevent lamp blackening. It is interesting to note that not only does this patent not teach lamp life enhancement through the use of carbon monoxide, but in fact teaches just the opposite, that is, that lamp life falls off rapidly with increasing carbon monoxide content.
In U.S. Pat. No. 3,728,572 to Maier et al., which issued on Apr. 17, 1973, a halogen incandescent lamp is described. As in the case of the '376 patent, the fill gas comprises carbon monoxide. In Maier et al., the carbon monoxide is used to decrease the level of halocarbon halogen additive needed while maintaining substantially equal lamp life.
In U.S. Pat. No. 4,163,171 to Wurster, which issued on July 31, 1979, a halogen cycle incandescent lamp is described. This patent refers to the operation of such a lamp wherein various materials diffuse out of the glass into the lamp bulb to adversely affect the regenerative cycle. It is noted that the release of carbon monoxide, carbon dioxide, and water during operation of the lamp will speed up the regenerative cycle. This patent essentially relates to high purity lamp glass which will avoid influencing the halogen cycle.
It is interesting to note that none of the known prior art relates to the use of carbon monoxide to increase lamp life. Similarly, none of the known prior art relates to any affect of carbon monoxide other than in the fill gas. It is an object of the present invention to provide an improved fabricating process in which carbon monoxide is used as a cleanup gas during lamp manufacture to improve lamp life and overcome the problems discussed herein. The carbon monoxide gas of the present invention is not used in any way as a fill gas.